what event leads to the final showdown between Buck and Spitz?

One of the newest dogs, Dolly, becomes mad and attacks Buck. Buck flees and eventually races back, relying on Francois to save him. Francois kills Dolly with an axe, and Buck collapses, exhausted. Spitz takes advantage of this moment of weakness and attacks. He is driven off by Francois who administers a severe lashing. Perrault believes that Spitz is going to kill Buck, but Francois insists that it is Spitz who should be afraid.

Everyone seems to know that the fight is drawing nearer. Buck wants it. He craves it because it is in his nature. Like the others, he has learned the pride of toil, and this pride threatens Spitz. He interferes when Spitz disciplines other dogs. He cares more about undermining Spitz than anything, and once he is soundly punished by Francois for protecting a malingerer from Spitz's punishment. The solidarity of the group is destroyed by Buck's clever undermining of Spitz's authority.